Perth in summer 2010/11: Very hot summer in Perth

Rainfall

Rainfall in the Perth metropolitan area during summer 2010/11 was generally
above average, with totals for the three months mainly in the 30 to 50 mm range.
A tropical low off the west coast that brought flooding rain to the Gascoyne
caused moderate rainfall in the Perth region on 20 and 21 December. Moderate
rainfall was also reported across the city in the 24 hours to 9am on 6 January
2011 due to a cold front and middle level cloudband, while a line of thunderstorms,
originating from the north, crossed the Perth Metropolitan area during the afternoon
of 29 January 2011 causing moderate rainfall at several locations. Rainfall
during February 2011 in the Perth area was generally below average, with several
sites observing no rainfall for the month, however, this is not uncommon for
this time of year.

Severe thunderstorms developed during the afternoon of 28 February 2011 and
produced heavy rainfall in northern, eastern and southern parts of the Perth
Metropolitan area during the afternoon. Rainfall totals in the 24 hours to 9am
on 1 March 2011 (which was likely to have fallen in the afternoon of 28 February)
observed at Bureau of Meteorology weather stations were 19.2 mm at Champion
Lakes, 19.0 mm at Gosnells City, 9.6 mm at Jarrahdale, 4.2 mm at Bickley and
0.4 mm at Serpentine. Hourly data at Champion Lakes and Bickley indicate that
Champion Lakes observed 19.2 mm between 1628 and 1800 WST on the 28 February,
while Bickley observed 4.2 mm between 1556 and 1800 WST.

Rottnest Island recorded 80.0 mm during summer 2010/11, almost three times
the summer average, and the 8th highest summer rainfall total since records
commenced in 1879. The highest summer rainfall at Rottnest Island is 180.9 mm
in the 1954/55 summer.

Perth Metro received 48.0 mm on 12 rain days over summer 2010/11, above the
average of 35.0 mm on 8.2 rain days. Individual monthly totals for summer 2010/11
were: 17.4 mm in December (average 12.8 mm); 30.2 mm in January (wettest since
the 139.0 mm in 2000; average 9.5 mm); and 0.4 mm in February (average 12.7
mm). The wettest day at Perth Metro during summer 2010/11 was 13.6 mm observed
in the 24 hours to 9am on 6 January 2011. Last summer (2009/10), Perth Metro
recorded 0.2 mm on 1 rain days, which was the driest summer since records commenced
in 1876.

Maximum temperature

After a cooler than normal start to summer 2010/11, mean daily maximum temperatures
were mostly warmer than normal, with most sites 1 to 2 °C above normal and
a few sites more than 2 °C above normal. Perth Metro's mean daily maximum
temperature in summer 2010/11 was 32.0 °C, which is 1.9 °C above average
and equal hottest summer on record with 1977/78 and 2009/10. Swanbourne and
Medina Research Centre also broke their hottest summer on record in terms of
daytime temperatures.

Heatwave conditions (three or more days of 35.0 °C or more) were evident
at many sites around the Perth area from 25 to 27 December 2010, with the hottest
day of summer 2010/11 occurring during this period when Pearce RAAF observed
42.1 °C on the 27th. January 2011 also saw heatwaves from the 1st to 4th
and from the 27th to 30th January 2011, which helped Perth Metro record its
7th hottest January on record. Consistently warm conditions were experienced
throughout much of February 2011, particularly in the latter half of the month
when very hot conditions were experienced due to a broad surface trough over
much of WA and another trough off the west coast dragging hot and humid northeasterly
winds over Perth. Heatwave conditions were observed at most Perth stations during
this time, with daily maximum temperatures generally in excess of 35 °C
from the 21st through to the 28th, with little to no relief provided by the
sea breeze. Perth Metro observed its 3rd highest February mean daily maximum
temperaure.

During summer 2010/11, Perth Metro observed a total of 60 days when the maximum
temperature had been 30 °C or higher, breaking the previous record of 56
days observed in summer 2009/10. The average for the number of days at 30 °C
or higher in summer is 44.5 days at Perth Metro, from observations beginning
in 1897. Perth Metro also observed 28 days when the maximum temperature was
35 °C or higher in summer, which is the 3rd highest, behind the record of
31 days in 1977/78 and the 29 days in 1961/62. The average for the number of
days 35 °C or higher in summer is 18 days.

Daily maxima during summer 2010/11 at Perth Metro ranged from a mild 21.1 °C
on 3 December 2010 to a very hot 40.4 °C on 29 January 2011.

Minimum temperature

Minimum temperatures across Perth during summer 2010/11 were very much above average,
with a number of stations breaking their highest summer mean daily minimum records.
Most sites were 1 to 2 °C above normal and a few sites more than 2 °C above normal.
Apart from the first few weeks of December 2010, overnight summer temperatures across Perth were consistently above average,
particularly during the second half of February 2011 where nearly all sites observed minimum temperatures
of over 20 °C from the 14th through to the end of the month.

Perth Metro observed 15 consecutive nights with a maximum temperature of 20 °C
or more during February 2011 (14th to 28th) which is the longest hot spell of this kind,
breaking the record of 13 nights observed in 1985 (17 February to 1 March) and in 1990 (23 February to 7 March).

Perth Metro's mean daily minimum temperature in summer 2010/11 was 19.0 °C,
which is the highest since the 19.2 °C in 1991/92. The average for summer
is 17.3 °C. Daily minima at Perth Metro during summer 2010/11 ranged from
a coo1 9.4 °C on 4 December 2010 to a hot 26.1 °C on 28 January 2011.

Wind and Sunshine

Very strong easterly winds were observed in the Perth area on 26 December 2010,
and on 5 and 6 February 2011 which were associated with significant bushfires
at Redhill and Roleystone/Kelmscott.

Perth’s average daily sunshine hours during summer 2010/11 was 11.0 hours,
close to the average of 11.4 hours.

Other Phenomena

The average dew-point temperature, a measure of the moisture content of the air
(the temperature to which air - at constant atmospheric pressure - must be cooled in order for dew to occur)
at Perth Metro during summer2010/11 was 13.6 °C, which is 1.0 °C above the average and lead to uncomfortable
conditions across Perth for much of summer 2010/11, particularly during January and February.

Severe thunderstorms developed in the afternoon of 28 February in the north, east and south of Perth in a hot,
moist and unstable atmosphere.
The worst affected suburbs were Ellenbrook, Middle Swan, Midland, Mundaring and Kalamunda,
with reports of damage, localised flooding and loss of power.

Notes

A Seasonal Climate Summary is prepared to list the main features of the weather in Perth using the most timely and accurate information available on the date of publication; it will generally not be updated. Later information, including data that has had greater opportunity for quality control, will be presented in the Monthly Weather Review, usually published in the fourth week of the month.

This statement has been prepared based on information available at
11 am on Thursday 3 March 2011.
Some checks have been made on the data, but it is possible that results will change
as new information becomes available.

Averages are long-term means based on observations from all available
years of record, which vary widely from site to site. They are not shown for
sites with less than 10 years of record.
The median
is sometimes more representative than the
mean
of long-term average rain.

The Rank indicates how rainfall this time compares with the climate record for the site,
based on the
decile ranking
(very low rainfall is in decile 1, low in decile 2 or 3,
average in decile 4 to 7, high in decile 8 or 9
and very high is in decile 10).
The Fraction of average shows how much rain has fallen this time as a
percentage of the long-term mean.

Records in this summary are based on sites with at least 10 years of data.

*Official Perth observations have been recorded at several different sites
with varying instrumentation over the years, and these changes can affect
the continuity of the climate record. For the current Mt Lawley site, the
most significant changes of this kind relate to minimum temperature and rain
days.

For that reason, current mean temperature and rain-day data quoted here are
based only on readings from the Mt Lawley site, which commenced observations
in 1993. Sunshine recordings are from Perth Airport and for similar reasons
current means use only Perth Airport data, which commenced in 1993.

For PDF files of Perth Metro climate statistics for other months,
please click here
for extremes and here
for averages.